1. Field of the Invention
The present invention relates to a dot-on-demand type ink jet printer including piezoelectric elements capable of reliably printing high quality images at high speed.
2. Related Art
There has been proposed a dot-on-demand type image forming device. Although the dot-on-demand type image forming device is relatively slow in printing speed compared with a continuous type image forming device, the dot-on-demand type image forming device has a simple configuration, so has become more popular.
Japanese Patent Application Publication (Kokai) No. HEI-11-78013 discloses a dot-on-demand line-scanning type ink jet recording device including a print head. The print head has a width corresponding to an entire width of a recording sheet, and is formed with a plurality of nozzles arranged in a line. Each nozzle is provided with an ejection element, such as a piezoelectric element or thermal element. The ejection elements are selectively driven based on a print signal while the recording sheet is being transported in a sheet feed direction at a high speed. As a result, ink droplets are ejected from the nozzles and hit on corresponding scanning lines of the recording sheet. In this way, ink images are formed on the recording sheet.
In this type of image forming device, because each nozzle of the print head corresponds to each one of scanning lines on the recording sheet, a large number of nozzles are necessary. For example, in order to form an image on a recording sheet having an 18-inch width at a resolution of 300 dot/inch (dpi), 5,400 (300 dpi×18 inch) nozzles need to be formed to the print head. In order to form the image with four different colors, 21,600 (5,400 nozzles×4 colors) nozzles are necessary.
However, it is difficult and expensive to produce an accurate print head with such a large number of nozzles without causing unevenness among the nozzles. Uneven nozzles undesirably degrade printing quality. Moreover, even if a precise print head is produced, unevenness may occur among the nozzles over time of use.
Specifically, unevenness among nozzles will cause the following problems. FIG. 1 is a top view showing a print head 207 and a recording sheet 406. The print head 207 is fixed at a predetermined position and ejects ink against the recording sheet 406 while the recording sheet 406 is being transported in a direction indicated by an arrow y with respect to the print head 207. In FIG. 1, dot regions on the recording sheet 406 are indicated by broken lines. Because the printer is designed for 300 dpi resolution in the x direction, each dot region has a width of 85 μm in the x direction. The print head 207 has formed dots 401 through 405 in every other dot regions on the recording sheet 406. The dot 401 is formed in a suitable manner. However, the dots 402 through 405 are formed at in an undesirable manner.
That is, the dot 402 is formed slightly above the target dot region. One possible explanation for this is that an ink droplet corresponding to the dot 402 is ejected from the print head 207 at an ejection speed higher than a proper ejection speed. Details will be described while referring to FIG. 2.
As described above, the recording sheet 406 is being transported in the y direction with respect to the print head 207 when the ink droplet is ejected. Therefore, although the ink droplet is ejected at the time when a position YO of the recording sheet 406 is located directly beneath a corresponding nozzle of the print head 207, an actual location where the ejected ink droplet impacts is a position Y which is different from the ejection position YO. The impact position Y is determined in a following equation:Y=YO−D×Vp/Vd  (E1)
wherein Y is the position where the ink droplet impacts;
Y0 is the position which is located directly beneath the corresponding nozzle when the ink droplet is ejected from the nozzle;
D is a distance between the nozzle and the recording sheet 406;
Vp is a transporting speed of the recording sheet 406 in the y direction; and
Vd is an average ejection speed of the ink droplet.
That is, when the ejection speed Vd is higher than a desired ejection speed, then a dot is recorded above a desired impact position in FIG. 1. On the other hand, when the ejection speed Vd is slower than the desired ejection speed, then a dot is recorded below the target impact position.
FIG. 1, the dot 403 has a smaller diameter than the dot 401. Such a dot is formed when an ink amount of a corresponding ink droplet is insufficient. The dot 404 has an elongate shape in the Y direction. When an ink droplet being ejected has a higher ejection speed at its leading portion than the ejection speed at its tailing portion, then the ink droplet impacts onto the recording sheet 406 while having an elongate shape rather than a circular shape. This results in forming a dot having an unusual dot shape, such as the dot 404. The dot 405 is called satellite dot which has a larger dot and a smaller dot formed below and separated from the larger dot. The satellite dot is formed when speed difference between a leading portion and a tailing portion of an ejected ink droplet is greater than that of the dot 404. That is, an ink droplet being ejected is divided into two or more droplets before the ink droplet impacts on the recording sheet 406 because of the speed difference. When recorded dots include these unusual dots, quality of images will be undesirably degraded. Such problems occur in any type of on-demand ink jet printer regardless of which type of ink or nozzles are used.